Tuning Built-In Voltage with Carbazole Self-Assembled Monolayers in Vacuum-Processed Organic Solar Cells

Katherine Trinkaus^a, Markus Perle^a, Hugo Norris^a, Pascal Kaienburg^a, Mathias Nyman^b, Oskar Sandberg^b, Moritz Riede^a

^aDepartment of Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford, OX1 3PU, United Kingdom

^bAbo Akademi University, Porthaninkatu 3, Turku, 20500, Finland

Proceedings of MATSUS Spring 2026 Conference (MATSUSSpring26)

B1 Future of Organic solar cells: What is next?

Barcelona, Spain, 2026 March 23rd - 27th

Organizers: Vida Engmann, Karen Forberich and Pascal Kaienburg

Oral, Katherine Trinkaus, presentation 423
Publication date: 15th December 2025

Self-assembled monolayers (SAMs) have previously been shown to perform well as hole-selective layers in solution-processed organic solar cells (OSCs) [1-5]. In this work, we investigated the feasibility of several common spin-coated carbazole SAMs – 2PACz, MeO-2PACz, and Br-2PACz, spanning a range of HOMO levels and work functions from about -4.5 to -6 eV – for integration in vacuum processed fullerene-based OSCs with DTDCPB or DCV4T as donor molecules, with HOMO energies of approximately –5.4 and –5.8 eV respectively. We found that certain SAMs increased the power conversion efficiency of the DTDCPB and DCV4T OSCs by up to 20% and 13% respectively relative to that of our control devices with a standard MoO_x hole contact: this relative enhancement in performance was linked to improvements in open circuit voltage (V_OC) which occurred if the HOMO of the SAM was below that of the donor. We used capacitance voltage measurements to directly determine the built-in voltage (V_bi) of each OSC and demonstrated that the trend in V_biclosely follows the V_OC and the energetic alignment of the different SAMs relative to each donor. These results can be used to help guide future SAM-donor combinations in OSCs. As vacuum thermal evaporation is a readily-scalable fabrication method [6], we seek to make fully evaporated SAM-based OSCs. In preparation for this, we attempted to evaporate the aforementioned SAMs. We have so far found that Br-2PACz degraded during attempted evaporation, but we were able to successfully evaporate 2PACz, in agreement with literature [7]. In this presentation, we aim to compare the fully evaporated device stacks to their solution processed counterparts. We will also complete complementary characterisation to confirm the same energetic changes occur, and we plan to attempt stability testing on these devices.

References:

[1] Kim, J. S.; Park, J. H.; Lee, J. H.; Jo, J.; Kim, D.-Y.; Cho, K. Control of the Electrode Work Function and Active Layer Morphology via Surface Modification of Indium Tin Oxide for High Efficiency Organic Photovoltaics. Appl. Phys. Lett. 2007, 91 (11), 112111.

[2] Lin, Y.; Firdaus, Y.; Isikgor, F. H.; Nugraha, M. I.; Yengel, E.; Harrison, G. T.; Hallani, R.; El-Labban, A.; Faber, H.; Ma, C.; Zheng, X.; Subbiah, A.; Howells, C. T.; Bakr, O. M.; McCulloch, I.; Wolf, S. D.; Tsetseris, L.; Anthopoulos, T. D. Self-Assembled Monolayer Enables Hole Transport Layer-Free Organic Solar Cells with 18% Efficiency and Improved Operational Stability. ACS Energy Lett. 2020, 5 (9), 2935–2944.

[3] Bin, H.; Datta, K.; Wang, J.; van der Pol, T. P. A.; Li, J.; Wienk, M. M.; Janssen, R. A. J. Finetuning Hole-Extracting Monolayers for Efficient Organic Solar Cells. ACS Appl. Mater. Interfaces 2022, 14 (14), 16497–16504.

[4] Lee, K. C.; Kim, S.; Son, J.; Kong, J.; Yang, C. A Brief Review on Self-Assembled Monolayers in Organic Solar Cells: Progress, Challenges, and Future Prospects. ACS Appl. Electron. Mater. 2025, 7 (3), 946–963.

[5] Azeez, A.; Huang, Y.; Stanly, L.; Kan, Z.; Karuthedath, S. Advances in Self-Assembled Monolayer-Engineered Organic Solar Cells. EES Solar 2025, 1 (3), 248–266.

[6] Riede, M.; Spoltore, D.; Leo, K. Organic Solar Cells—The Path to Commercial Success. Advanced Energy Materials 2021, 11 (1), 2002653.

[7] Farag, A.; Feeney, T.; Hossain, I. M.; Schackmar, F.; Fassl, P.; Küster, K.; Bäuerle, R.; Ruiz-Preciado, M. A.; Hentschel, M.; Ritzer, D. B.; Diercks, A.; Li, Y.; Nejand, B. A.; Laufer, F.; Singh, R.; Starke, U.; Paetzold, U. W. Evaporated Self-Assembled Monolayer Hole Transport Layers: Lossless Interfaces in p-i-n Perovskite Solar Cells. Advanced Energy Materials 2023, 13 (8), 2203982.

Acknowledgements:

K. T. is funded by the Clarendon Scholarship, awarded in partnership with the Waverley Scholarship and she thanks the Clarendon Fund; The Queen’s College, Oxford; and the Waverley Fund for their support. We acknowledge the EPSRC National Thin Film Facility for Advanced Functional Materials (NTCF), hosted by the Department of Physics at the University of Oxford, and Dr Jin Yao and Dr Matthew Naylor, the facility engineers for their support. The NTCF was funded by ESPRC (EP/M022900/1), the Wolfson Foundation and the University of Oxford. H. N. acknowledges the EPSRC for funding his Vacation Internship. P. K. thanks EPSRC for funding for a Postdoctoral Fellowship (no. EP/V035770/1) and Linacre College for the award of a Junior Research Fellowship. O.J.S. acknowledges funding from the Research Council of Finland through Project No. 357196. M. N. acknowledges funding from The Swedish Cultural Foundation in Finland.

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